G-protein-coupled receptor (GPCR)-mediated regulation of the GTPases Rho and Rac is a conserved signaling module required for vital cell behaviors. However, the mechanisms that connect GPCRs, and their heterotrimeric G-protein (G?/?/?) partners, to Rho/Rac are not fully delineated. Our studies in C. elegans and human endothelial cells have revealed a new conserved player required for Rho/Rac signaling: the Chloride Intracellular Channel (CLIC) family of proteins. Our data shows that CLICs are required in two important endothelial GPCR pathways (S1P/S1P Receptor and thrombin/PAR) that function through G?12/13 and G?i to activate Rho and Rac. This CLIC function is evolutionarily conserved, because we found that in C. elegans the CLIC ortholog exc-4 genetically interacts with the G?12 ortholog gpa-12 and with the Rac orthologs ced-10 and mig-2. The molecular function of CLICs has long remained a mystery. Based on sequence and structural similarities, they have been proposed to function as chloride channels and/or as glutathione S-transferases (GST). Previous work has shown that EXC-4 membrane localization is mediated by an N-terminal domain and this localization is critical for function. We have now found that CLIC membrane localization is also critical for its role in GPCR-mediated Rho/Rac activation in endothelial cells, and that replacement of the membrane-targeting N-terminus with a myristoylation signal is sufficient to restore this function. Since the channel and GST activities of CLICs require an intact N- terminus we have discovered a novel activity for CLICs. We hypothesize that CLICs are membrane-localized regulators of Rho and Rac that respond to GPCR-G? signaling. In Aim 1 we will determine how CLICs couple GPCR-G? (G?12/13 and G?i) signaling to Rho and Rac. We will survey the requirement for CLICs in different cell and signaling contexts to define key GPCR-G? combinations that utilize CLICs to regulate of Rho and Rac. We will use cutting-edge bio-sensors and genetic tools to measure and modulate signaling to determine which step in the GPCR-G???-Rho/Rac cascade requires CLICs. Finally, we will test whether CLICs physically interact with Rho/Rac to modulate signaling. In Aim 2 we will define the determinants by which CLICs regulate Rho/Rac in human cells and in C. elegans by performing structure-function analyses, focused on the EXC-4/CLIC C- terminus. Critical domains defined in this Aim will be tested for their ability to interact with Rho/Rac (as defined in Aim 1). In Aim 3 we will carry out unbiased genetic and proteomic screens in C. elegans to find conserved players that genetically and physically interact with EXC-4/CLIC to further elucidate how CLICs regulate Rho/Rac signaling. We will then test whether human orthologs of genes identified in these screens influence Rho/Rac signaling in mammalian cells. By defining new mechanisms of action for CLICs in GPCR-G???-Rho/Rac signaling we will significantly increase our knowledge of how GPCRs influence human biology and uncover new ways of targeting these pathways.